1. Field of the Invention
The present invention relates to methods and apparatuses for suppressing cross-coloration in video displays, and more particularly, the present invention teaches a method of adjusting a chrominance signal based on a luminance transition derived from high-frequency components of a luminance signal and a related apparatus.
2. Description of the Prior Art
With technological advances in display technology, video processing, and integrated circuit fabrication, in tandem with the rapid development of wireless networking, users can view their favorite movies and television programs on a video display device (such as a television) any time, any place. Thus, information and entertainment become increasingly accessible, and user requirements for picture quality increase in like manner.
The human eye has four different types of light receptor, of which three are used for distinguishing light of different wavelength (the fourth is only used under dim lighting conditions, and cannot discern colors). In other words, all light visible to the human eye can be fully described by three axes. Thus, when displaying a picture, only red, blue, and green (RGB) light information output is needed, when speaking in terms of the human eye, to show an image of realistic quality. However, to reduce bandwidth and ensure compatibility, the prior art color television broadcast system does not directly output RGB signals, but instead outputs a composite signal. The “composite” signal is an output signal that is a mix of a luminance signal and a chrominance signal, which is compatible with black-and-white and color television systems, and also conserves bandwidth.
The earliest television was the black-and-white television. Later, when color television systems were being developed, to promote compatibility between black-and-white television signals and color television signals, black-and-white (luminance) signals and color (chrominance) signals were separated. In this way, a black-and-white television needed only to decode the incoming luminance signal from a television station in order to display a picture. Color televisions would decode both the luminance signal and the chrominance signal together in order to display a color picture. Because the human eye is more sensitive to luminance than chrominance, or in other words, the human eye requires less color resolution than black-and-white resolution, the color signal does not require as much bandwidth as the black-and-white signal. Thus, by taking advantage of the human eye's relative insensitivity to color, transmission bandwidth can be reduced and used in black-and-white and color televisions.
Taking the National Television Standards Committee (NTSC) standard as an example, NTSC originally used a YIQ color space. The YIQ color space uses quadrature modulation to synthesize a common spectrum intermodulation signal I with a quadrature signal Q to form a single chrominance signal C. The chrominance signal C is then added to a luminance signal Y, and with an accompanying horizontal synchronization pulse, a blanking pulse, and a color burst, the composite signal is generated. The NTSC standard adopts a 6 MHz channel bandwidth, with 4.2 MHz reserved for the luminance signal Y, 1.6 MHz given to the intermodulation signal I, and 0.6 MHz appropriated to the quadrature signal Q. In contrast to the NTSC standard signal, the Phase Alternating Line (PAL) standard adopts a YUV color space. To increase picture quality, a color phase of the chrominance signal is alternately set as positive and negative for each successive scanline. The PAL standard uses an 8 MHz channel, allocating 5.5 MHz to the luminance signal Y and 1.8 MHz to a signal U and a signal V.
Thus, by splitting the luminance signal and the chrominance signal, then transmitting the signals together, the transmission bandwidth can be reduced, and the transmitted signal can be used in both black-and-white and color televisions. Correspondingly, a receiving end need only comprise a circuit such as a comb filter, for isolating the luminance signal Y and the chrominance signal C, in order to play both black-and-white and color television. However, the composite Y/C signal has one large problem, which primarily lies in the fact that high-frequency components of the luminance signal Y overlap with the frequency spectrum of the chrominance signal C. This makes it difficult for the receiving end to accurately and completely separate the luminance signal Y and the chrominance signal C in their original forms from the composite signal Y/C. Ultimately, this inability to separate the luminance signal Y from the chrominance signal C results in flaws in the picture. For example, if the luminance signal Y is processed as part of the chrominance signal C, a cross-color artifact is produced, and the picture will exhibit a rainbow effect. Likewise, if the chrominance signal C is processed as part of the luminance signal Y, a cross-luma artifact is produced, resulting in a horizontal or vertical dotted line in the picture.
In order to reduce the effects of the high-frequency components of the luminance signal Y occupying the same frequency spectrum as the chrominance signal C, U.S. Pat. No. 6,108,048 teaches a method of using a low pass filter to get a low-frequency portion of the luminance signal Y, and adjusting the chrominance signal C based on determination of a diagonal edge by detecting a luminance transition between scanlines of neighboring frames. In other words, the prior art uses the low-frequency portion of the luminance signal Y as a basis for finding the diagonal edge. However, because the low-frequency signal is more susceptible to low-frequency noise, the prior art often makes incorrect judgments, which means that the prior art is often unable to remove cross-coloration accurately. Further, when the incorrect judgment occurs, without a corresponding compensation mechanism, the picture quality is adversely affected.